The present invention is directed to improvements in the squealer tips of high temperature turbine airfoil components.
Components operating in the turbine portion of a gas turbine engine and within the gas flow path of the combustor portion of the engine experience severe a environmental conditions. These parts are subject to significant temperature extremes, approaching the melting temperature of some of the components. In addition, the hot gases of combustion from which energy for operation of the engine is derived are both corrosive and oxidative. Although the alloys that are used in these extreme conditions were especially developed to operate in them, environmental coatings and/or thermal barrier coatings are often applied to the external surfaces of these components to both protect the alloy from the harsh environment and to allow the alloy to withstand higher operating temperatures. Environmental coatings are generally oxidation resistant metallic coatings applied over the structural component; while thermal barrier coating systems are comprised of an underlying bond coat, which can be an environmental coating, and an overlying ceramic thermal barrier layer. Of course, the achievement of higher operating temperatures is an overall objective, as higher operating temperatures generally improve the efficiency of the engine.
The exhaust gases from the combustor are directed through the turbine in such a manner that the hottest gases impinge turbine airfoils, and specifically turbine blades, at or near their tip. In order to extract the maximum amount of energy from the hot stream of exhaust gases, the clearance between the blade tips and the adjacent shrouds is kept at a minimum. However, as a result of dimensional tolerancing stack-up during manufacturing operations and thermal expansion of components during hot operations in the initial cycles of engine operations, as well as certain anticipated events such as occasional hard landings, the tips of the airfoils can severely rub into the shroud. Although the shrouds are designed to be abradable to account for such anticipated rubbing, often the rubbing can adversely affect the blade tip, causing the removal of the outer coating from the tip region. As the outer coating is either a thermal barrier coating or an environmental coating, the exposure of the underlying material produces undesirable results. When a thermal barrier coating is removed by robust contact with the adjacent shroud, the underlying bond coat is exposed to elevated temperatures very early in the life of the engine that it was not designed for. When an environmental coating is removed by robust contact with the adjacent shroud, the underlying superalloy base material is exposed to hot oxidative and corrosive gases very early in the life of the engine from which it was to be protected by the environmental coating.
The consequences of this excess rubbing can be immediate, in that the engine efficiency will be affected by hot gases escaping around the gap between the airfoil and the shroud. As the blade deteriorates more quickly over time as a result of the loss of the overlying protective coating, the engine efficiency will be further reduced. And naturally, the life of the engine will be shortened due to more rapid degradation of the blades.
Various solutions have been attempted to solve the problem of squealer tip wear and deterioration. One illustrative example is set forth in U.S. Pat. No. 5,622,638 to Schell et al. and assigned to the assignee of the present invention. Schell""s approach is to apply a special Ni-base alloy as a repair over squealer tips that have been machined in preparation for repair. Many of the solutions deal with repairing cast airfoils removed from service in order to restore their environmentally damaged and worn tips. A similar solution is set forth in U.S. Pat. No. 4,822,248 in which seals are repaired with wear resistant material using plasma arc welding in order to minimize the amount of base metal that is heated to form a metallurgical bond. The patent also discloses adding a wear resistant material to the notch or flat portion adjacent to the cast seal portion of the blade for new blades.
Another approach is set forth in U.S. Pat. No. 5,048,183 to Cang et al. which discloses a method of adding a tip to a turbine blade by weld depositing using weld layered puddling to add a material of predetermined strength less than the blade base metal strength and that is resistant to oxidation and corrosion.
What the prior art lacks is a turbine blade having a squealer tip made of a material that is resistant to oxidation, but has a predetermined strength at least as strong or stronger than the blade base material. While corrosion resistance is not an unimportant property for a squealer tip, it is not as critical as oxidation. Because airfoils are either directionally solidified or single crystal, it is necessary to add the material to the turbine blade in a manner that has a minimal impact on the base material, that is to say, the added material should be added in a manner that minimizes the melting of the underlying base material.
The present invention is directed to overcome the shortcomings of the prior art.
A squealer tip is added to an otherwise conventionally cast new turbine airfoil. Allowances are made in the casting so that the final airfoil meets product dimensional requirements after the squealer tip is added. The squealer tip is comprised of an alloy that is different from the cast alloy of the airfoil in that it has a predetermined strength that is greater than the cast alloy material and has oxidation resistance that is superior to those of the cast blade, making it more capable of surviving for long periods in the harsh environment of a gas turbine engine. Because the material added to the new airfoil by the squealer tip can be varied, sufficient material can be added by the processes of the present invention to assure that rubbing against the mating turbine shroud occurring during initial engine breaking does not consume all of the added squealer tip. The squealer tip thus can be added so that sufficient environmentally protective material remains after engine break-in to provide environmental protection to the airfoil as the corrosive and oxidative hot gases of combustion flow past the airfoil tip.
In one embodiment, the squealer tip is added to the airfoil casting by a process that minimizes the mixing of the airfoil alloy and the added squealer tip, but which provides a strong mechanical bond and a weak or partial metallurgical bond. To fully develop the metallurgical bond between the airfoil casting and the squealer tip, the airfoil is heat treated for a preselected time at an elevated temperature below the melting point of either alloy but sufficiently high to permit formation of a full metallurgical bond. Excess material may be added to the airfoil casting, which is machined back so that the airfoil with attached blade is within the design tolerances of the drawing.
An advantage of the present invention is that the airfoil can be made of two different alloys, with the alloy exposed to the highest temperatures and the most severe environmental conditions having properties that will extend its life under these conditions. Because alloys capable of withstanding these temperature extremes and severe environmental conditions are more expensive and typically more difficult to cast, the restricted use of the alloy only in the region where these conditions are most severe can reduce the cost of manufacturing the airfoil.
Another advantage of the present invention is that it permits the application of the squealer tip made from a second high temperature, high strength and oxidation resistant alloy to an airfoil casting while minimizing mixing of the two alloys without the concomitant reduction of properties when alloys are joined by processes that involve mixing, such as conventional welding techniques.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.